In apparatuses for recording/reproducing optical information, higher densities can be attained by using a light source of a shorter wavelength. For example, in compact disk (CD) players, which have been popular for some time, near-infrared light of 780 nm wavelength is used, whereas digital versatile disk (DVD) players, in which information is reproduced at higher densities, use a red semiconductor laser of 650 nm wavelength. In order to realize the next-generation disk players with even higher densities, research in blue laser light sources of even shorter wavelengths is intense. In order to realize a blue laser light source that is compact and stable, wavelength conversion elements using non-linear optical materials are being researched, and for devices using non-linear optical crystals, there are quasi-phase-matching optical waveguide-type wavelength conversion devices.
One approach to realize optical waveguide-type wavelength conversion devices is the ridge-type optical waveguide device shown in FIG. 12. This optical waveguide device 48 is made of a ridge portion 52 and periodic polarization inversion regions 51 provided on an X-cut Mg-doped LiNbO3 substrate 49. The refractive index of the ridge portion 52 is higher than the refractive index around it, so that the vicinity of the ridge portion 52 functions as an optical waveguide 50 and light is guided inside this optical waveguide 50. Laser light that is guided inside the optical waveguide 50 is converted into light of ½ the wavelength. The optical waveguide device 48 provided with the ridge portion 52 can use the crystal itself as a waveguide layer, so that the problem of deterioration of the non-linearity, which occurs in optical waveguides using conventional ion-exchange, does not occur, and a highly efficient wavelength conversion can be achieved. Thus, using the ridge-type optical waveguide device 48, a violet optical output of 410 nm wavelength can be obtained from red input light of 820 nm wavelength.
A method for manufacturing the ridge-type optical waveguide device is described briefly with reference to FIG. 13. First, as shown in FIG. 13A, an optical substrate 1 is laminated to the surface of a substrate 2 of about 1 mm thickness using an ultraviolet (UV) curing resin 3. Then, as shown in FIG. 13B, the surface of the optical substrate 1 is abraded. Here, the optical substrate 1, which is laminated to the substrate 2, is abraded to a thickness of 3.5 μm. Finally, grooves 4 are formed in the surface of the abraded optical substrate 1 by laser processing, as shown in FIG. 13C. Thus, a ridge-type optical waveguide 5 is obtained, finishing the ridge-type optical waveguide device.
However, in order to form the ridge-type optical waveguide, it is necessary to control the thickness of the optical substrate 1 in the abrasion step with high precision, because if the thickness of the optical substrate is not controlled at high precision, the light in the ridge-type waveguide is not guided anymore. Yet in the actual abrasion step, the thickness of the optical substrate 1 could not be controlled precisely, resulting in thickness variations of ±1 μm or more, and the yield was very poor. Furthermore, since the abrasion was carried out while measuring the thickness of the optical substrate 1 in order to perform abrasion at high precision, considerable time was needed for the formation of the ridge-type optical waveguide. In addition, control of the abrasion time was necessary.
There were the following further problems in the fabricated optical waveguide devices.
When using optical wavelength conversion elements utilizing an optical waveguide, a highly efficient wavelength conversion becomes possible, but in order to improve the conversion efficiency and realize high power characteristics, it is necessary to propagate the guided wave at high power densities within the optical waveguide. For example, in order to obtain second harmonic generation (SHG) light of several 10 mW, at least twice the power is required for the fundamental wave. At present, light sources used in optical disk devices produce short wavelengths of several 10 mW, and even higher powers are required. Moreover, a high power guided wave is required for the optical waveguide device itself For example, new applications in optical waveguide switches and modulators used for communication or sensors can be developed by raising the power of the guided wave.
However, if the power of the guided wave propagated in the optical waveguide is increased, the temperature of the optical waveguide is increased due to absorption of the guided wave. A guided wave of several mW is not all that problematic, but in the case of a guided wave of several 10 mW, even a slight absorption can lead to a large temperature increase in optical waveguides with high power densities. The problem of temperature increase due to absorption of the guided wave becomes even more serious with shorter wavelengths. The inventors of the present invention have found in the course of developing high-power optical wavelength conversion elements that a temperature increase of the optical waveguide may become a cause for a deterioration of the power of the optical wavelength conversion element. Furthermore, the inventors of the present invention also found that when the power of the guided wave is increased in conventional optical waveguide devices, the temperature of the waveguide layer increases due to absorption of the guided wave, which leads to the problem of deteriorated characteristics and shorter device lifetime.
It is thus an object of the present invention to provide a method for manufacturing an optical waveguide device in which the thickness of an optical substrate can be controlled with high precision when manufacturing the optical waveguide device, and a shortening of the manufacturing time can be achieved. It is a further object of the present invention to provide an optical waveguide device, in particular a ridge-type optical waveguide device as necessitated by high-power guided waves, as well as a coherent light source and an optical apparatus using this optical waveguide device, with which a deterioration of the characteristics caused by temperature increases due to absorption of the guided wave can be avoided, and even if the guided wave is absorbed, the resulting temperature increase can be kept low and the temperature distribution can be made uniform, thus achieving more stable characteristics.